FIG. 2 shows an example of a tandem printing system in which two printers P1 and P2 are tandemly arranged in order to create images on both sides of a web which is illustrated as a long, continuous belt-like paper. That is, a web W is fed to the printer P1 in the first stage and images are formed on its first surface (main surface) by an image-forming apparatus 19. And, after the web W has been turned over by a turn bar T and then fed into the printer P2 in the subsequent stage, images are formed by an image-forming apparatus 20 on the second surface (back-side surface) of the web W. Thus, this is a double-side printing system.
FIG. 3 shows an example of a spot-color tandem printing system in which the printing with a first color is executed by printer P1 in the first stage and then the printing with a second color is executed by printer P2 in the subsequent stage.
The present invention relates to a web conveyance method and apparatus of those tandem printing systems.
FIG. 4 shows the outline of conveying the web in the tandem printing system.
A web W which has on its first surface an image created by printer P1 is discharged from the printer P1 by a carrier roller 15. The web W is guided to printer P2 via a guide roller 8, a guide roller 9, and a guide roller 10 which constitute a turn bar T disposed subsequently after the printer P1. The web W is then fed into a web feeding mechanism 23 via a guide roller 7 and a guide roller 22; and after an air loop 24 has been formed, the web W is conveyed to an image-forming apparatus 20 by a carrier roller 21. After an image has been formed on the second surface of the web W by the image-forming apparatus 20, the web W is discharged from the printer P2 by a carrier roller 25.
Herein, the carrier roller 15 of the printer P1, web feeding mechanism 23 of the printer P2, carrier roller 21, and the carrier roller 25 are all capable of conveying the web W. On the other hand, the guide rollers 8 to 10 which constitute a turn bar T and the guide rollers 7 and 22 located at the entrance of the printer P2 are driven rollers having no conveyance capability or unrotatable, immovable rollers.
In the above-mentioned tandem printing system, the guide roller 22 and guide rollers 7 to 10, located upstream of the web feeding mechanism 23, create a load on the web W during the printing operation, which prevents slack in the web along the path of the web as shown as W22b and W10b. 
When the printing operation is stopped, the inertia of the web W itself or the inertia of the rollers disposed on the web conveyance path causes the web to be conveyed too much, which creates slack W22a and slack W10a between the two printers P1 and P2. For example, if the printing operation begins in the state where slack W22a is present as shown in the drawing, the portion of the web located downstream of the web feeding mechanism 23 accelerates, while the portion of the web upstream of the guide roller 22 remains stationary. If acceleration of the portion of the web W located downstream of the web feeding mechanism 23 has stopped before slack W22a disappears; at the moment when slack W22a disappears, the portion of the web located upstream of the guide roller 22 is momentarily accelerated at the highest speed. For this reason, a great deal of impulse is to be imposed on the portion of the web between the web feeding mechanism 23 and the guide roller 22. At this time, if the web feeding mechanism 23 has a weak conveyance force, an error which eliminates the air loop 24 will stop printing, and if the web W is thin, the web W may break. The same phenomena will occur in and around slack W10a. 
To avoid such phenomena, for example, as disclosed in Japanese patent laid-open No. 2004-292133, a method in which a web is conveyed early by a web feeding mechanism 23 has been proposed.